Role of HIF-1alpha in skeletal development

Hyperbaric Oxygen Therapy Improves the Osteogenic and Vasculogenic Properties of Mesenchymal Stem Cells in the Presence of Inflammation In Vitro

Hyperbaric oxygen (HBO) therapy has been reported to be beneficial for treating many conditions of inflammation-associated bone loss. The aim of this work was to in vitro investigate the effect of HBO in the course of osteogenesis of human Mesenchymal Stem Cells (MSCs) grown in a simulated pro-inflammatory environment. Cells were cultured with osteogenic differentiation factors in the presence or not of the pro-inflammatory cytokine Tumor Necrosis Factor-α (TNF-α), and simultaneously exposed daily for 60 min, and up to 21 days, at 2,4 atmosphere absolute (ATA) and 100% O₂. To elucidate osteogenic differentiation-dependent effects, cells were additionally pre-committed prior to treatments. Cell metabolic activity was evaluated by means of the MTT assay and DNA content quantification, whereas osteogenic and vasculogenic differentiation was assessed by quantification of extracellular calcium deposition and gene expression analysis. Metabolic activity and osteogenic properties of cells did not differ between HBO, high pressure (HB) alone, or high oxygen (HO) alone and control if cells were pre-differentiated to the osteogenic lineage. In contrast, when treatments started contextually to the osteogenic differentiation of the cells, a significant reduction in cell metabolic activity first, and in mineral deposition at later time points, were observed in the HBO-treated group. Interestingly, TNF-α supplementation determined a significant improvement in the osteogenic capacity of cells subjected to HBO, which was not observed in TNF-α-treated cells exposed to HB or HO alone. This study suggests that exposure of osteogenic-differentiating MSCs to HBO under in vitro simulated inflammatory conditions enhances differentiation towards the osteogenic phenotype, providing evidence of the potential application of HBO in all those processes requiring bone regeneration.

Integrin-specific hydrogels modulate transplanted human bone marrow-derived mesenchymal stem cell survival, engraftment, and reparative activities

Stem cell therapies are limited by poor cell survival and engraftment. A hurdle to the use of materials for cell delivery is the lack of understanding of material properties that govern transplanted stem cell functionality. Here, we show that synthetic hydrogels presenting integrin-specific peptides enhance the survival, persistence, and osteo-reparative functions of human bone marrow-derived mesenchymal stem cells (hMSCs) transplanted in murine bone defects. Integrin-specific hydrogels regulate hMSC adhesion, paracrine signaling, and osteoblastic differentiation in vitro. Hydrogels presenting GFOGER, a peptide targeting α2β1 integrin, prolong hMSC survival and engraftment in a segmental bone defect and result in improved bone repair compared to other peptides. Integrin-specific hydrogels have diverse pleiotropic effects on hMSC reparative activities, modulating in vitro cytokine secretion and in vivo gene expression for effectors associated with inflammation, vascularization, and bone formation. These results demonstrate that integrin-specific hydrogels improve tissue healing by directing hMSC survival, engraftment, and reparative activities.

Type H blood vessels in bone modeling and remodeling

In the mammalian skeletal system, osteogenesis and angiogenesis are intimately linked during bone growth and regeneration in bone modeling and during bone homeostasis in bone remodeling. Recent studies have expanded our knowledge about the molecular and cellular mechanisms responsible for coupling angiogenesis and bone formation. Type H vessels, termed such because of high expression of Endomucin (Emcn) and CD31, have recently been identified and have the ability to induce bone formation. Factors including platelet-derived growth factor type BB (PDGF-BB), slit guidance ligand 3 (SLIT3), hypoxia-inducible factor 1-alpha (HIF-1α), Notch, and vascular endothelial growth factor (VEGF) are involved in the coupling of angiogenesis and osteogenesis. This review summarizes the current understanding of signaling pathways that regulate type H vessels and how type H vessels modulate osteogenesis. Further studies dissecting the regulation and function of type H vessels will provide new insights into the role of bone vasculature in the metabolism of the skeleton. We also discuss considerations for therapeutic approaches targeting type H vessels to promote fracture healing, prevent pathological bone loss, osteonecrosis, osteoarthritis, and bone metastases.

HIF-1α-TWIST pathway restrains cyclic mechanical stretch-induced osteogenic differentiation of bone marrow mesenchymal stem cells

Aim: Mechanical strain plays a crucial role in bone formation and remodeling. Hypoxia-inducible factor (HIF)-1α and TWIST are upstream of master regulators of osteogenesis, including runt-related transcription factor 2 (RUNX2) and bone morphogenetic proteins (BMPs). This study investigated the effect of the HIF-1α-TWIST pathway on cyclic mechanical stretch-induced osteogenic differentiation of rat bone marrow mesenchymal stem cells (BMSCs) and the underlying mechanism. Materials and Methods: BMSCs were isolated from bone marrow derived from the femurs and humeri of Sprague-Dawley rats. Osteogenic differentiation of BMSCs was induced by applying cyclic mechanical stretch using the Flexcell Tension System. HIF-1α and TWIST were knocked down using recombinant lentiviral vectors. Osteogenic differentiation was evaluated by real-time qPCR, western blotting, and the alkaline phosphatase (ALP) activity assay. Results: Cyclic mechanical stretch increased ALP activity and expression of HIF-1α and TWIST in BMSCs. Knockdown of HIF-1α decreased TWIST expression in stretched BMSCs. Moreover, knockdown of HIF-1α or TWIST enhanced cyclic mechanical stretch-induced osteogenic differentiation of BMSCs. In addition, knockdown of TWIST increased expression of RUNX2 and BMP2 in stretched BMSCs. Conclusions: The HIF-1α-TWIST signaling pathway inhibits cyclic mechanical stretch-induced osteogenic differentiation of BMSCs. This finding may facilitate cell and tissue engineering for clinical applications.

Hypoxia-inducible factor (HIF): how to improve osseointegration in hip arthroplasty secondary to avascular necrosis in sickle cell disease

Many studies in the literature have been carried out to evaluate the various cellular and molecular processes involved in osteogenesis.

Angiogenesis and bone formation work closely together in this group of disorders. Hypoxia-inducible factor (HIF) which is stimulated in tissue hypoxia triggers a cascade of molecular processes that helps manage this physiological deficiency.

However, there still remains a paucity of knowledge with regard to how sickle cell bone pathology, in particular avascular necrosis, could be altered when it comes to osseointegration at the molecular level.

Hypoxia-inducible factor has been identified as key in mediating how cells adapt to molecular oxygen levels.

The aim of this review is to further elucidate the physiology of hypoxia-inducible factor with its various pathways and to establish what role this factor could play in altering the disease pathophysiology of avascular necrosis caused by sickle cell disease and in improving osseointegration.

This review article also seeks to propose certain research methodology frameworks in exploring how osseointegration could be improved in sickle cell disease patients with total hip replacements and how it could eventually reduce their already increased risk of undergoing revision surgery.

Cite this article: EFORT Open Rev 2019;4:567-575. DOI: 10.1302/2058-5241.4.180030

Expression of HIF‑1α in cycling stretch‑induced osteogenic differentiation of bone mesenchymal stem cells

During orthodontic treatment, mechanical force is applied to the teeth, and following a series of complex metabolism changes, the position of the teeth in the alveolar bone change. This process is closely associated with primitive bone mesenchymal stem cells (BMSCs), which may differentiate into osteoblasts precursor cell. A hypoxic microenvironment may be caused by orthodontic mechanical forces between the alveolar bone and the root. Hypoxia-inducible factor 1α (HIF-1α) is a specific receptor that adapts to a hypoxic environment. The present study was designed to investigate whether HIF-1α was involved in the osteoblastic differentiation of BMSCs induced by cyclic tensile stress. During this process, HIF-1α mRNA and protein expression were detected using a reverse transcription-quantitative polymerase chain reaction and western blotting. It was revealed that alkaline phosphatase activity increased in a time-dependent manner in three different stretching strength groups, which indicates that cyclic stretch promotes the osteogenic differentiation of BMSCs. The optimal force stage of osteogenesis was an unexpected discovery, which will provide theoretical guidance for selecting the most suitable orthodontic force for tooth movement in clinical orthodontic treatment. Most importantly, all experiments revealed that HIF-1α mRNA and protein were significantly increased following stretching treatment in BMSCs. It was therefore concluded that HIF-1α may be involved in BMSCs modulating osteogenic metabolism during exposure to cyclic stretch and a hypoxic microenvironment, which may prove useful for the reconstruction of a jaw during orthodontic treatment.

Tuftelin and HIFs expression in osteogenesis

Tuftelin was originally discovered and mostly studied in the tooth, but later found also in other organs. Despite its wide distribution among tissues, tuftelin's function has so far been specified only in the formation of enamel crystals. Nevertheless, in many cases, tuftelin was suggested to be associated with cellular adaptation to hypoxia and recently even with cell differentiation. Therefore, we aimed to investigate tuftelin expression along with hypoxia-inducible factors (HIFs) during the early development of the mandibular/alveolar (m/a) bone, when osteoblasts started to differentiate in vivo and to compare their expression levels in undifferentiated versus differentiated osteoblastic cells in vitro. Immunohistochemistry demonstrated the presence of tuftelin already in osteoblastic precursors which were also HIF1-positive, but HIF2-negative. Nevertheless, HIF2 protein appeared when osteoblasts differentiated, one day later. This is in agreement with observations made with MC3T3-E1 cells, where there was no significant difference in tuftelin and Hif1 expression in undifferentiated vs. differentiated cells, although Hif2 increased upon differentiation induction. In differentiated osteoblasts of the m/a bone, all three proteins accumulated, first, prenatally, in the cytoplasm and later, particularly at postnatal stages, they displayed also peri/nuclear localization. Such a dynamic time-space pattern of tuftelin expression has recently been reported in neurons, which, as the m/a bone, differentiate under less hypoxic conditions as indicated also by a prevalent cytoplasmic expression of HIF1 in osteoblasts. However, unlike what was shown in cultured neurons, tuftelin does not seem to participate in final osteoblastic differentiation and its functions, thus, appears to be tissue specific.

Supercritical CO2 foamed composite scaffolds incorporating bioactive lipids promote vascularized bone regeneration via Hif-1α upregulation and enhanced type H vessel formation

Bone tissue engineering has substantial potential for the treatment of massive bone defects; however, efficient vascularization coupled with bone regeneration still remains a challenge in this field. In the current study, supercritical carbon dioxide (scCO₂) foaming technique was adopted to fabricate mesoporous bioactive glasses (MBGs) particle-poly (lactic-co-glycolic acid) (PLGA) composite scaffolds with appropriate mechanical and degradation properties as well as in vitro bioactivity. The MBG-PLGA scaffolds incorporating the bioactive lipid FTY720 (designated as FTY/MBG-PLGA) exhibited simultaneously sustained release of the bioactive lipid and ions. In addition to providing a favorable microenvironment for cellular adhesion and proliferation, FTY/MBG-PLGA scaffolds significantly facilitated the in vitro osteogenic differentiation of rBMSCs and also markedly stimulated the upregulation of Hif-1α expression via the activation of the Erk1/2 pathway, which mediated the osteogenic and pro-angiogenic effects on rBMSCs. Furthermore, FTY/MBG-PLGA extracts induced superior in vitro angiogenic performance of HUVECs. In vivo evaluation of critical-sized rat calvarial bone defects indicated that FTY/MBG-PLGA scaffolds potently promoted vascularized bone regeneration. Notably, the significantly enhanced formation of type H vessels (CD31^hiEmcn^hi neo-vessels) was observed in newly formed bone tissue in FTY/MBG-PLGA group, strongly suggesting that FTY720 and therapeutic ions released from the scaffolds synergistically induced more type H vessel formation, which indicated the coupling of angiogenesis and osteogenesis to achieve efficiently vascularized bone regeneration. Overall, the results indicated that the foamed porous MBG-PLGA scaffolds incorporating bioactive lipids achieved desirable vascularization-coupled bone formation and could be a promising strategy for bone regenerative medicine. STATEMENT OF SIGNIFICANCE: Efficacious coupling of vascularizationandbone formation is critical for the restoration of large bone defects. Anoveltechnique was used to fabricate composite scaffolds incorporating bioactive lipids which possessedsynergistic cues of bioactive lipids and therapeutic ions to potently promotebone regenerationas well as vascularization. The underlying molecular mechanism for the osteogenic and pro-angiogenic effects of the compositescaffolds was unveiled. Interestingly, the scaffolds were furtherfoundto enhance the formation oftype H capillarieswithin the bone healing microenvironment to couple angiogenesis to osteogenesis to achieve satisfyingvascularizedbone regeneration.These findings provide a novel strategy to develop efficiently vascularized engineering constructs to treat massive bone defects.

The wonders of BMP9: From mesenchymal stem cell differentiation, angiogenesis, neurogenesis, tumorigenesis, and metabolism to regenerative medicine

Although bone morphogenetic proteins (BMPs) initially showed effective induction of ectopic bone growth in muscle, it has since been determined that these proteins, as members of the TGF-β superfamily, play a diverse and critical array of biological roles. These roles include regulating skeletal and bone formation, angiogenesis, and development and homeostasis of multiple organ systems. Disruptions of the members of the TGF-β/BMP superfamily result in severe skeletal and extra-skeletal irregularities, suggesting high therapeutic potential from understanding this family of BMP proteins. Although it was once one of the least characterized BMPs, BMP9 has revealed itself to have the highest osteogenic potential across numerous experiments both in vitro and in vivo, with recent studies suggesting that the exceptional potency of BMP9 may result from unique signaling pathways that differentiate it from other BMPs. The effectiveness of BMP9 in inducing bone formation was recently revealed in promising experiments that demonstrated efficacy in the repair of critical sized cranial defects as well as compatibility with bone-inducing bio-implants, revealing the great translational promise of BMP9. Furthermore, emerging evidence indicates that, besides its osteogenic activity, BMP9 exerts a broad range of biological functions, including stem cell differentiation, angiogenesis, neurogenesis, tumorigenesis, and metabolism. This review aims to summarize our current understanding of BMP9 across biology and the body.

Muscle injury-induced hypoxia alters the proliferation and differentiation potentials of muscle resident stromal cells

Background

Trauma-induced heterotopic ossification (HO) is a complication that develops under three conditions: the presence of an osteogenic progenitor cell, an inducing factor, and a permissive environment. We previously showed that a mouse multipotent Sca1⁺ CD31⁻ Lin⁻ muscle resident stromal cell (mrSC) population is involved in the development of HO in the presence of inducing factors, members of the bone morphogenetic protein family. Interestingly, BMP9 unlike BMP2 causes HO only if the muscle is damaged by injection of cardiotoxin. Because acute trauma often results in blood vessel breakdown, we hypothesized that a hypoxic state in damaged muscles may foster mrSCs activation and proliferation and trigger differentiation toward an osteogenic lineage, thus promoting the development of HO.

Methods

Three- to - six-month-old male C57Bl/6 mice were used to induce muscle damage by injection of cardiotoxin intramuscularly into the tibialis anterior and gastrocnemius muscles. mrSCs were isolated from damaged (hypoxic state) and contralateral healthy muscles and counted, and their osteoblastic differentiation with or without BMP2 and BMP9 was determined by alkaline phosphatase activity measurement. The proliferation and differentiation of mrSCs isolated from healthy muscles was also studied in normoxic incubator and hypoxic conditions. The effect of hypoxia on BMP synthesis and Smad pathway activation was determined by qPCR and/or Western blot analyses. Differences between normally distributed groups were compared using a Student’s paired t test or an unpaired t test.

Results

The hypoxic state of a severely damaged muscle increased the proliferation and osteogenic differentiation of mrSCs. mrSCs isolated from damaged muscles also displayed greater sensitivity to osteogenic signals, especially BMP9, than did mrSCs from a healthy muscle. In hypoxic conditions, mrSCs isolated from a control muscle were more proliferative and were more prone to osteogenic differentiation. Interestingly, Smad1/5/8 activation was detected in hypoxic conditions and was still present after 5 days, while Smad1/5/8 phosphorylation could not be detected after 3 h of normoxic incubator condition. BMP9 mRNA transcripts and protein levels were higher in mrSCs cultured in hypoxic conditions. Our results suggest that low-oxygen levels in damaged muscle influence mrSC behavior by facilitating their differentiation into osteoblasts. This effect may be mediated partly through the activation of the Smad pathway and the expression of osteoinductive growth factors such as BMP9 by mrSCs.

Conclusion

Hypoxia should be considered a key factor in the microenvironment of damaged muscle that triggers HO.

Electronic supplementary material

The online version of this article (10.1186/s13395-019-0202-5) contains supplementary material, which is available to authorized users.

HIF-1α facilitates osteocyte-mediated osteoclastogenesis by activating JAK2/STAT3 pathway in vitro

Osteocytes, entrapped within the mineralized bone matrix, has been found to have numerous functions such as acting as an orchestrator of bone remodeling through regulation of both osteoclast and osteoblast activity and also functioning as an endocrine cell. Due to a specialized morphology and surrounding structure, osteocytes are more tolerant to hypoxia during osteoporosis, fracture, osteoarthritis, and orthodontic-orthognathic combination therapy. Hypoxia-inducible factor-1α (HIF-1α) is one of the master regulators of hypoxia reactions, playing an important role in bone modeling, remodeling, and homeostasis. This study aimed to investigate the pivotal functional role of HIF-1α in osteocytes initiating of bone remodeling under hypoxia. In the present study, the osteoclasts formation induced by RAW264.7 was significantly promoted in conditioned media (CM) from osteocytic MLO-Y4 exposed to hypoxia in vitro. Therefore, hypoxic MLO-Y4 cells simulated by 100 μmol/L CoCl₂ or 2% O₂ stably expressed HIF-1α proteins and upregulated the expression of receptor activator of nuclear factor-κB ligand (RANKL) at both the messenger RNA (mRNA) and protein level. Furthermore, with the Knockdown of HIF-1α, the expression of RANKL mRNA and protein decreased after transient transfection. In addition, the phosphorylation of Janus kinase 2 (JAK2) and signal transducer and activator of transcription (STAT3) was also correlated with HIF-1α and RANKL levels under hypoxia. Then AG490, a JAK2 inhibitor, inhibited p-JAK2, p-STAT3 and RANKL expression. It was possible that AG490 disturbed the contact of HIF-1α and RANKL by JAK2/STAT3 pathway, influencing osteoclastogenesis. Our findings suggested that HIF-1α promoted the expression of RANKL by activating JAK2/STAT3 pathway in MLO-Y4 cells, and enhanced osteocyte-mediated osteoclastic differentiation in vitro.

Gain-of-function mutation of microRNA-140 in human skeletal dysplasia

MicroRNAs (miRNAs) are post-transcriptional regulators of gene expression. Heterozygous loss-of-function point mutations of miRNA genes are associated with several human congenital disorders^1-5, but neomorphic (gain-of-new-function) mutations in miRNAs due to nucleotide substitutions have not been reported. Here we describe a neomorphic seed region mutation in the chondrocyte-specific, super-enhancer-associated MIR140 gene encoding microRNA-140 (miR-140) in a novel autosomal dominant human skeletal dysplasia. Mice with the corresponding single nucleotide substitution show skeletal abnormalities similar to those of the patients but distinct from those of miR-140-null mice⁶. This mutant miRNA gene yields abundant mutant miR-140-5p expression without miRNA-processing defects. In chondrocytes, the mutation causes widespread derepression of wild-type miR-140-5p targets and repression of mutant miR-140-5p targets, indicating that the mutation produces both loss-of-function and gain-of-function effects. Furthermore, the mutant miR-140-5p seed competes with the conserved RNA-binding protein Ybx1 for overlapping binding sites. This finding may explain the potent target repression and robust in vivo effect by this mutant miRNA even in the absence of evolutionary selection of miRNA-target RNA interactions, which contributes to the strong regulatory effects of conserved miRNAs^7,8. Our study presents the first case of a pathogenic gain-of-function miRNA mutation and provides molecular insight into neomorphic actions of emerging and/or mutant miRNAs.

HIF-1α Regulates Glucocorticoid-Induced Osteoporosis Through PDK1/AKT/mTOR Signaling Pathway

Long-term and high dose glucocorticoid treatment can cause decreased viability and function of osteoblasts, which leads to osteoporosis and osteonecrosis. In this study, we investigated the role and mechanism of action of HIF-1α in glucocorticoid-induced osteogenic inhibition in MC3T3-E1 cells. Our results showed that HIF-1α protein expression was reduced when MC3T3-E1 cells were exposed to dexamethasone (Dex) at varying concentrations ranging from 10^-9 to 10^-6 M. PDK1 expression was also decreased in MC3T3-E1 cells after dexamethasone treatment. MC3T3-E1 cells when treated with the glucocorticoid receptor antagonist RU486 along with dexamethasone showed enhanced HIF-1α expression. In addition, upregulated expression of HIF-1α was capable of promoting the osteogenic ability of MC3T3-E1 cells and PDK1 expression. However, the HIF-1α antagonist 2-methoxyestradiol (2-ME) had a reverse effect in MC3T3-E1 cells exposed to dexamethasone. Furthermore, the PDK1 antagonist dichloroacetate could repress the osteogenic ability of MC3T3-E1 cells, although HIF-1α was upregulated when transduced with adenovirus-HIF-1α construct. The PDK1 agonist PS48 was able to promote the osteogenic ability of MC3T3-E1 cells treated with dexamethasone. Importantly, the protein levels of p-AKT and p-mTOR were increased in MC3T3-E1 cells treated with dexamethasone after PS48 treatment. in vivo, the PDK1 agonist PS48 could maintain the bone mass of mice treated with dexamethasone. This study provides a new understanding of the mechanism of glucocorticoid-induced osteoporosis.

3D-porous β-tricalcium phosphate-alginate-gelatin scaffold with DMOG delivery promotes angiogenesis and bone formation in rat calvarial defects

Hypoxia-inducible factor-1α (HIF-1α), a well-studied angiogenesis pathway, plays an essential role in angiogenesis-osteogenesis coupling. Targeting the HIF-1a pathway frequently leads to successful reconstruction of large-sized bone defects through promotion of angiogenesis. Dimethyloxalylglycine (DMOG) small molecule regulates the stability of HIF-1α at normal oxygen tension by mimicking hypoxia, which subsequently accelerates angiogenesis. The current study aims to develop a novel construct by seeding adipose derived mesenchymal stem cells (ADMSCs) onto a scaffold that contains DMOG to induce angiogenesis and regeneration of a critical size calvarial defect in a rat model. The spongy scaffolds have been synthesized in the presence and absence of DMOG and analyzed in terms of morphology, porosity, pore size, mechanical properties and DMOG release profile. The effect of DMOG delivery on cellular behaviors of adhesion, viability, osteogenic differentiation, and angiogenesis were subsequently evaluated under in vitro conditions. Histological analysis of cell-scaffold constructs were also performed following transplantation into the calvarial defect. Physical characteristics of fabricated scaffolds confirmed higher mechanical strength and surface roughness of DMOG-loaded scaffolds. Scanning electron microscopy (SEM) images and MTT assay demonstrated the attachment and viability of ADMSCs in the presence of DMOG, respectively. Osteogenic activity of ADMSCs that included alkaline phosphatase (ALP) activity and calcium deposition significantly increased in the DMOG-loaded scaffold. Computed tomography (CT) imaging combined with histomorphometry and immunohistochemistry analysis showed enhanced bone formation and angiogenesis in the DMOG-loaded scaffolds. Therefore, spongy scaffolds that contained DMOG and had angiogenesis ability could be utilized to enhance bone regeneration of large-sized bone defects.

Active mitochondria support osteogenic differentiation by stimulating β-catenin acetylation

Bone marrow stromal (a.k.a. mesenchymal stem) cells (BMSCs) can differentiate into osteoblasts (OBs), adipocytes, or chondrocytes. As BMSCs undergo OB differentiation, they up-regulate mitochondrial oxidative phosphorylation (OxPhos). Here, we investigated the mechanism(s) connecting mitochondrial OxPhos to OB differentiation. First, we found that treating BMSC-like C3H10T1/2 cells with an OxPhos inhibitor reduces their osteogenic potential. Interestingly, ATP levels were not reduced, as glycolysis compensated for the decreased OxPhos. Thus, mitochondria support OB differentiation not only by supplying ATP, but also by other mechanisms. To uncover these mechanisms, we stimulated OxPhos in C3H10T1/2 cells by replacing media glucose with galactose and observed that this substitution increases both OxPhos and osteogenesis even in the absence of osteoinducers. β-Catenin, an important signaling pathway in osteogenesis, was found to be responsive to OxPhos stimulation. β-Catenin activity is maintained by acetylation, and mitochondria generate the acetyl donor acetyl-CoA, which upon entering the Krebs cycle is converted to citrate capable of exiting mitochondria. Cytosolic citrate is converted back to acetyl-CoA by ATP citrate lyase (ACLY). We found that inhibiting ACLY with SB204990 (SB) reverses the galactose-induced β-catenin activity and OB differentiation. This suggested that acetylation is involved in β-catenin activation after forced OxPhos stimulation, and using immunoprecipitation, we indeed detected SB-sensitive β-catenin acetylation. Both β-catenin acetylation and activity increased during osteoinduction coincident with OxPhos activation. These findings suggest that active mitochondria support OB differentiation by promoting β-catenin acetylation and thus activity.

Osteocytic oxygen sensing controls bone mass through epigenetic regulation of sclerostin

Preservation of bone mass is crucial for healthy ageing and largely depends on adequate responses of matrix-embedded osteocytes. These cells control bone formation and resorption concurrently by secreting the WNT/β-catenin antagonist sclerostin (SOST). Osteocytes reside within a low oxygen microenvironment, but whether and how oxygen sensing regulates their function remains elusive. Here, we show that conditional deletion of the oxygen sensor prolyl hydroxylase (PHD) 2 in osteocytes results in a high bone mass phenotype, which is caused by increased bone formation and decreased resorption. Mechanistically, enhanced HIF-1α signalling increases Sirtuin 1-dependent deacetylation of the Sost promoter, resulting in decreased sclerostin expression and enhanced WNT/β-catenin signalling. Additionally, genetic ablation of PHD2 in osteocytes blunts osteoporotic bone loss induced by oestrogen deficiency or mechanical unloading. Thus, oxygen sensing by PHD2 in osteocytes negatively regulates bone mass through epigenetic regulation of sclerostin and targeting PHD2 elicits an osteo-anabolic response in osteoporotic models.

Osteocytes reside in a low oxygen environment, but it is not clear if oxygen sensing regulates their function. Here, the authors show that deletion of the oxygen sensor prolyl hydroxylase 2 in osteocytes leads to increased bone mass via regulation of sclerostin, and reduces bone loss in mouse models of osteoporosis.

Hypoxia regulates angeogenic-osteogenic coupling process via up-regulating IL-6 and IL-8 in human osteoblastic cells through hypoxia-inducible factor-1α pathway

Inappropriate angiogenesis and osteogenesis are considered as the crucial factors of osteoporotic fracture. Hypoxia is a primary driving force for regulating the angiogenic-osteogenic coupling process. Our recent results indicated that hypoxia could improve angiogenesis as well as differentiation and activity of osteoblastic cells via up-regulating VEGF through HIF-1α pathway. Here we demonstrated that in human osteoblastic MG-63, U2-OS and Saos-2 cells, besides VEGF, the other two pro-angiogenic factors IL-6 and IL-8 were also up-regulated by hypoxia and CoCl₂ (a mimic of hypoxia). Mechanism studies indicated overexpression of HIF-1α (generated from transfection with a plasmid encoding sense HIF-1α) markedly increased the levels of IL-6 and IL-8 in osteoblastic cells. Furthermore, a luciferase reporter assay was performed using the reporter vector containing the IL-6 or IL-8 promoter sequence to illustrate observably increased activity of hypoxia-induced IL-6 and IL-8 promoter caused by overexpression of HIF-1α. Additionally, chromatin immune-precipitation analysis showed hypoxia increased the DNA binding ability of HIF-1α to IL-6 or IL-8 promoter. Analysis in vitro by MTT test and Boyden chamber assay showed exogenous IL-6 and IL-8 (a relatively short period of treatment with recombinant IL-6 or IL-8 equivalent to the autocrine levels) could significantly promote the proliferation of human osteoblastic, endothelial and monocytic cells, as well as the migration of human endothelial cells. Taken together, these results indicate that IL-6 and IL-8 in osteoblastic cells may also contribute to the angiogenic-osteogenic coupling process via HIF-1α pathway. Besides VEGF, IL-6- or IL-8-targeted adjunctive therapy maybe a new strategy to improve the treatment of osteoporosis.

Identification of key gene networks associated with fracture healing using αSMA‑labeled progenitor cells

The aim of the present study was to investigate the key gene network in fracture healing. The dataset GSE45156 was downloaded from the Gene Expression Omnibus. Differentially expressed genes (DEGs) were identified using the linear models for microarray data package of Bioconductor. Subsequently, Gene Ontology (GO) functional and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analyses were conducted for DEGs in day 2 and 6 fractured samples via the Database for Annotation, Visualization and Integrated Discovery. Furthermore, protein-protein interactions (PPIs) of DEGs were analyzed and a PPI network was constructed. A total of 774 and 1,172 DEGs were identified in day 2 and 6 fractured samples, respectively, compared with unfractured controls. Of the DEGs in day 2 and 6 fractured samples, various upregulated DEGs, including protein kinase C α (Prkca) and B-cell lymphoma antagonist/killer 1 were significantly enriched in GO terms associated with cell death, and certain downregulated DEGs, including fms-related tyrosine kinase 1 (Flt1), nitric oxide synthase 3 (Nos3), bone morphogenetic protein 4 (Bmp4) and Notch1 were enriched in GO terms associated with angiogenesis. Furthermore, a series of downregulated DEGs were enriched in the Notch signaling pathway, including hes family bHLH transcription factor 1 and Notch1. Certain DEGs had a high degree and interacted with each other, including Flt1, Nos3, Bmp4 and Notch1, and Prkca and ras-related C3 botulinum toxin substrate 3. The up and downregulated DEGs may exert critical functions by interactively regulating angiogenesis or apoptosis.

Activation of hypoxia-inducible factor 1 attenuates periapical inflammation and bone loss

Hypoxia (low oxygen level) is an important feature during infections and affects the host defence mechanisms. The host has evolved specific responses to address hypoxia, which are strongly dependent on the activation of hypoxia-inducible factor 1 (HIF-1). Hypoxia interferes degradation of HIF-1 alpha subunit (HIF-1α), leading to stabilisation of HIF-1α, heterodimerization with HIF-1 beta subunit (HIF-1β) and subsequent activation of HIF-1 pathway. Apical periodontitis (periapical lesion) is a consequence of endodontic infection and ultimately results in destruction of tooth-supporting tissue, including alveolar bone. Thus far, the role of HIF-1 in periapical lesions has not been systematically examined. In the present study, we determined the role of HIF-1 in a well-characterised mouse periapical lesion model using two HIF-1α-activating strategies, dimethyloxalylglycine (DMOG) and adenovirus-induced constitutively active HIF-1α (CA-HIF1A). Both DMOG and CA-HIF1A attenuated periapical inflammation and tissue destruction. The attenuation in vivo was associated with downregulation of nuclear factor-κappa B (NF-κB) and osteoclastic gene expressions. These two agents also suppressed NF-κB activation and subsequent production of proinflammatory cytokines by macrophages. Furthermore, activation of HIF-1α by DMOG specifically suppressed lipopolysaccharide-stimulated macrophage differentiation into M1 cells, increasing the ratio of M2 macrophages against M1 cells. Taken together, our data indicated that activation of HIF-1 plays a protective role in the development of apical periodontitis via downregulation of NF-κB, proinflammatory cytokines, M1 macrophages and osteoclastogenesis.

Neurological heterotopic ossification: Current understanding and future directions

Neurological heterotopic ossification (NHO) involves the formation of bone in soft tissue following a neurological condition, of which the most common are brain and spinal cord injuries. NHO often forms around the hip, knee and shoulder joints, causing severe pain and joint deformation which is associated with significant morbidity and reduced quality of life. The cellular and molecular events that initiate NHO have been the focus of an increasing number of human and animal studies over the past decade, with this work largely driven by the need to unearth potential therapeutic interventions to prevent the formation of NHO. This review provides an overview of the present understanding of NHO pathogenesis and pathobiology, current treatments, novel therapeutic targets, potential biomarkers and future directions.

Effect of hypoxia on the proliferation of porcine bone marrow-derived mesenchymal stem cells and adipose-derived mesenchymal stem cells in 2- and 3-dimensional culture

OBJECTIVE

Bone marrow-derived mesenchymal stem cells (MSCs) and adipose-derived mesenchymal stem cells (ASCs) currently represent a promising tool for the regeneration of large bony defects. Therefore, it is pivotal to find the best cell source within the body and the best conditions for in vitro cellular expansion. This study compared cellular response of MSCs and ASCs from a porcine animal in normoxic (21% O2) and hypoxic (2% O2) cell culture conditions via 2D and 3D experimental settings.

MATERIALS AND METHODS

The effect of constant exposure to hypoxia on primary pig stem cells was evaluated by two methods. First, a cumulative population doublings (cumPD) over a period of 40 days, a metabolic activity assay in both 2D and 3D beta-TCP-PHB scaffolds, followed by analysis of osteogenic differentiation potential in cell monolayers.

RESULTS

Our results displayed enhanced cell culture proliferation in 2% O2 for both MSCs and ASCs, with impaired osteogenic differentiation of MSCs. The impact of constant hypoxia on porcine MSCs and ASCs exhibited a statistically significant decrease in osteogenic differentiation under hypoxic conditions with the MSCs.

CONCLUSIONS

Our data suggest that MSCs and ASCs expanded in hypoxic culture conditions, might be more suitable for use in the clinical setting where large cell numbers are required. When differentiated in normoxic conditions, MSCs showed the highest osteogenic differentiation potential and might be the best choice of cells with consideration to bone repair.

Proliferation and osteogenic differentiation of rat BMSCs on a novel Ti/SiC metal matrix nanocomposite modified by friction stir processing

The aims of this study were to fabricate a novel titanium/silicon carbide (Ti/SiC) metal matrix nanocomposite (MMNC) by friction stir processing (FSP) and to investigate its microstructure and mechanical properties. In addition, the adhesion, proliferation and osteogenic differentiation of rat bone marrow stromal cells (BMSCs) on the nanocomposite surface were investigated. The MMNC microstructure was observed by both scanning and transmission electron microscopy. Mechanical properties were characterized by nanoindentation and Vickers hardness testing. Integrin β1 immunofluorescence, cell adhesion, and MTT assays were used to evaluate the effects of the nanocomposite on cell adhesion and proliferation. Osteogenic and angiogenic differentiation were evaluated by alkaline phosphatase (ALP) staining, ALP activity, PCR and osteocalcin immunofluorescence. The observed microstructures and mechanical properties clearly indicated that FSP is a very effective technique for modifying Ti/SiC MMNC to contain uniformly distributed nanoparticles. In the interiors of recrystallized grains, characteristics including twins, fine recrystallized grains, and dislocations formed concurrently. Adhesion, proliferation, and osteogenic and angiogenic differentiation of rat BMSCs were all enhanced on the novel Ti/SiC MMNC surface. In conclusion, nanocomposites modified using FSP technology not only have superior mechanical properties under stress-bearing conditions but also provide improved surface and physicochemical properties for cell attachment and osseointegration.

Elevated concentrations of hypoxia-inducible factor-1α in patients with fracture and concomitant traumatic brain injury

Background Compelling evidence indicate that traumatic brain injury is highly related to accelerated bone fracture repair, but the underlying mechanism still remains elusive. Fracture repair process relies greatly on the formation of new blood vessels in fracture site, and angiogenic factors have been confirmed to be essential for the initiation and maintenance of the fracture healing. Hypoxia-inducible factor-1α was demonstrated to be a critical regulator of angiogenic-osteogenic coupling during bone development and regeneration. The aim of the present study was to investigate the local and circulating concentrations of hypoxia-inducible factor-1α in patients with long-bone fractures and concomitant traumatic brain injury and to determine the potential role of hypoxia-inducible factor-1α in fracture healing. Methods Twenty-five patients with a long-bone fracture and concomitant traumatic brain injury (FT group) and 33 without a brain injury (Fr group) were enrolled in this study. Healthy subjects donated serum samples as control. Serum samples were collected over a period of six months, following a standardized time schedule. Hypoxia-inducible factor-1α concentrations were measured in fracture haematoma and serum of patients in both groups using enzyme-linked immunosorbent assay. Results Patients in FT group had a short time to union. Serum hypoxia-inducible factor-1α concentrations elevated in the early healing period and reached the maximum level during intramembranous bone formation phase in both groups. Thereafter, it decreased continuously and approached to the minimum levels until the end of the observation period. Serum hypoxia-inducible factor-1α concentrations in both groups were significantly higher compared with controls and hypoxia-inducible factor-1α concentrations in both serum and fracture haematoma were higher in FT group than that in Fr group. Fracture haematoma contained significantly higher hypoxia-inducible factor-1α concentrations compared with hypoxia-inducible factor-1α concentrations in serum. Serum hypoxia-inducible factor-1α concentrations had a positive correlation with hypoxia-inducible factor-1α concentrations in fracture haematoma in patients with fractures. Conclusions These findings suggest the local and systemic involvement of hypoxia-inducible factor-1α in fracture healing and the accelerated fracture repair in patients with traumatic brain injury might be associated with elevated hypoxia-inducible factor-1α concentrations in fracture haematoma and serum.

Intermittent Hypoxia Influences Alveolar Bone Proper Microstructure via Hypoxia-Inducible Factor and VEGF Expression in Periodontal Ligaments of Growing Rats

Intermittent hypoxia (IH) recapitulates morphological changes in the maxillofacial bones in children with obstructive sleep apnea (OSA). Recently, we found that IH increased bone mineral density (BMD) in the inter-radicular alveolar bone (reflecting enhanced osteogenesis) in the mandibular first molar (M1) region in the growing rats, but the underlying mechanism remains unknown. In this study, we focused on the hypoxia-inducible factor (HIF) pathway to assess the effect of IH by testing the null hypothesis of no significant differences in the mRNA-expression levels of relevant factors associated with the HIF pathway, between control rats and growing rats with IH. To test the null hypothesis, we investigated how IH enhances mandibular osteogenesis in the alveolar bone proper with respect to HIF-1α and vascular endothelial growth factor (VEGF) in periodontal ligament (PDL) tissues. Seven-week-old male Sprague-Dawley rats were exposed to IH for 3 weeks. The microstructure and BMD in the alveolar bone proper of the distal root of the mandibular M1 were evaluated using micro-computed tomography (micro-CT). Expression of HIF-1α and VEGF mRNA in PDL tissues were measured, whereas osteogenesis was evaluated by measuring mRNA levels for alkaline phosphatase (ALP) and bone morphogenetic protein-2 (BMP-2). The null hypothesis was rejected: we found an increase in the expression of all of these markers after IH exposure. The results provided the first indication that IH enhanced osteogenesis of the mandibular M1 region in association with PDL angiogenesis during growth via HIF-1α in an animal model.

Selective enrichment of microRNAs in extracellular matrix vesicles produced by growth plate chondrocytes

Matrix vesicles (MVs) are membrane organelles found in the extracellular matrix of calcifying cells, which contain matrix processing enzymes and regulate the extracellular environment via action of these enzymes. It is unknown whether MVs are also exosomic mediators of cell-cell communication via transfer of RNA material, and specifically, microRNA (miRNA). We investigated the presence of RNA in MVs isolated from cultures of costochondral growth zone chondrocytes. Our results showed that the average yield of MV RNA was 1.93±0.78ng RNA/10(4) cells, which was approximately 0.1% of the parent cell's total RNA. MV RNA was well-protected from RNase by the lipid membrane and was highly enriched in small RNA molecules compared to cells. Moreover, coding and non-coding small RNAs in MVs were in proportions that differed from parent cells. Enrichment of specific miRNAs was consistently observed in all three miRNA detection platforms that we used, suggesting that miRNAs are selectively packaged into MVs. MV-enriched miRNAs were related to different signaling pathways associated with bone formation. This study suggests a significant role for MVs as "matrisomes" in cell-cell communication in cartilage and bone development via transfer of specific miRNAs.

Vitamin K2 Ameliorates Damage of Blood Vessels by Glucocorticoid: a Potential Mechanism for Its Protective Effects in Glucocorticoid-induced Osteonecrosis of the Femoral Head in a Rat Model

Glucocorticoid has been reported to decrease blood vessel number and harm the blood supply in the femoral head, which is recognized to be an important mechanism of glucocorticoid-induced osteonecrosis of the femoral head (ONFH). To prevent glucocorticoid-induced ONFH, medication that promotes both bone formation and angiogenesis would be ideal. Vitamin K2 has been revealed to play an important role in bone metabolism; however, few studies have focused on the effect of Vitamin K2 on new vascular formation. Thus, this study aimed to investigate whether Vitamin K2 promoted new blood vessel formation in the presence of glucocorticoids, both in vitro and in vivo. The effect of Vitamin K2 on viability, migration, in vitro tube formation, and VEGF, vWF, CD31, KDR, Flt and PDGFB in EAhy926 incubated with or without dexamethasone were elucidated. VEGF, TGF-β and BMP-2, angiogenesis-related proteins secreted by osteoblasts, were also detected in the osteoblast-like cell line of MG63. In addition, blood vessels of the femoral head in rats administered with or without methylprednisolone and Vitamin K2 were evaluated using angiography and CD31 staining. In vitro studies showed that Vitamin K2 significantly protected endothelial cells from dexamethasone-induced apoptosis, promoted endothelial cell migration and in vitro tube formation. Angiogenesis-related proteins both in EAhy926 and MG63 were also upregulated by Vitamin K2 when cotreated with dexamethasone. In vivo studies showed enhanced blood vessel volume and CD31-positive staining cells in rats cotreated with VK2 and methylprednisolone compared to rats treated with methylprednisolone only. Collectively, Vitamin K2 has the ability to promote angiogenesis in vitro and to ameliorate vessels of the femoral head in glucocorticoid-treated rats in vivo, indicating that Vitamin K2 is a promising drug that may be used to prevent steroid-induced ONFH.

Hypoxic regulation of osteoclast differentiation and bone resorption activity

Bone integrity is maintained throughout life via the homeostatic actions of bone cells, namely, osteoclasts, which resorb bone, and osteoblasts, which produce bone. Disruption of this balance in favor of osteoclast activation results in pathological bone loss, which occurs in conditions including osteoporosis, rheumatoid arthritis, primary bone cancer, and cancer metastasis to bone. Hypoxia also plays a major role in these conditions, where it is associated with disease progression and poor prognosis. In recent years, considerable interest has arisen in the mechanisms whereby hypoxia and the hypoxia-inducible transcription factors, HIF-1α and HIF-2α, affect bone remodeling and bone pathologies. This review summarizes the current evidence for hypoxia-mediated regulation of osteoclast differentiation and bone resorption activity. Role(s) of HIF and HIF target genes in the formation of multinucleated osteoclasts from cells of the monocyte-macrophage lineage and in the activation of bone resorption by mature osteoclasts will be discussed. Specific attention will be paid to hypoxic metabolism and generation of ATP by osteoclasts. Hypoxia-driven increases in both glycolytic flux and mitochondrial metabolic activity, along with consequent generation of mitochondrial reactive oxygen species, have been found to be essential for osteoclast formation and resorption activity. Finally, evidence for the use of HIF inhibitors as potential therapeutic agents targeting bone resorption in osteolytic disease will be discussed.

The pre-vascularisation of a collagen-chondroitin sulphate scaffold using human amniotic fluid-derived stem cells to enhance and stabilise endothelial cell-mediated vessel formation

A major problem in tissue engineering (TE) is graft failure in vivo due to core degradation in in vitro engineered constructs designed to regenerate thick tissues such as bone. The integration of constructs post-implantation relies on the rapid formation of functional vasculature. A recent approach to overcome core degradation focuses on the creation of cell-based, pre-engineered vasculature formed within the TE construct in vitro, prior to implantation in vivo. The primary objective of this study was to investigate whether an amniotic fluid-derived stem cell (AFSC)-human umbilical vein endothelial cell (HUVEC) co-culture could be used to engineer in vitro vasculature in a collagen chondroitin sulphate (CCS) scaffold. The secondary objective was to investigate whether hypoxic conditions (2% O2) could enhance microcapillary-like structure formation by this co-culture. The results of this study demonstrate, for the first time, that the AFSC-HUVEC co-culture was capable of pre-vascularising CCS scaffolds within 7 days and that the AFSCs are capable of behaving as pericytes while interacting with HUVECS to form microcapillary-like structures. However, this microcapillary-like structure formation was reduced in hypoxic conditions. qRT-PCR analysis indicated that an upregulation of VEGFR1 and accompanying decrease of VEGFR2 gene expression may be responsible for the poor response of these microcapillary-like structures to hypoxic conditions. Overall, however, these results demonstrate the potential of this newly developed co-culture system for the formation of pre-engineered vasculature within TE constructs.

STATEMENT OF SIGNIFICANCE

This article describes the development of an amniotic fluid-derived stem cell (AFSC)-human umbilical vein endothelial cell (HUVEC) co-culture for use in engineering in vitro vasculature in a collagen chondroitin sulphate (CCS) scaffold. The article also describes the effect of hypoxic conditions on the networks of microcapillary-like structures formed by this co-culture. The AFSC-HUVEC co-culture was capable of pre-vascularising CCS scaffolds within 7 days. However, microcapillary-like structure formation was reduced in hypoxic conditions. Overall, these results demonstrate the potential of this newly developed co-culture system for the formation of pre-engineered vasculature within TE constructs. The proangiogenic nature of this co-culture has the potential to both enhance bone regeneration while also overcoming the problem of inadequate vascularisation of grafts commonly seen in the field of tissue engineering.

Osteogenic capillaries orchestrate growth plate-independent ossification of the malleus

Endochondral ossification is a developmental process by which cartilage is replaced by bone. Terminally differentiated hypertrophic chondrocytes are calcified, vascularized, and removed by chondroclasts before bone matrix is laid down by osteoblasts. In mammals, the malleus is one of three auditory ossicles that transmit vibrations of the tympanic membrane to the inner ear. The malleus is formed from a cartilaginous precursor without growth plate involvement, but little is known about how bones of this type undergo endochondral ossification. Here, we demonstrate that in the processus brevis of the malleus, clusters of osteoblasts surrounding the capillary loop produce bone matrix, causing the volume of the capillary lumen to decrease rapidly in post-weaning mice. Synchrotron X-ray tomographic microscopy revealed a concentric, cylindrical arrangement of osteocyte lacunae along capillaries, indicative of pericapillary bone formation. Moreover, we report that overexpression of Fosl1, which encodes a component of the AP-1 transcription factor complex, in osteoblasts significantly blocked malleal capillary narrowing. These data suggest that osteoblast/endothelial cell interactions control growth plate-free endochondral ossification through ‘osteogenic capillaries’ in a Fosl1-regulated manner.

Summary: The endochondral ossification of the malleus, an ossicle of the mouse inner ear, occurs around capillaries and is mediated by the AP-1 transcription factor Fosl1.

Involvement of Prolyl Hydroxylase Domain Protein in the Rosiglitazone-Induced Suppression of Osteoblast Differentiation

Rosiglitazone is a well-known anti-diabetic drug that increases insulin sensitivity via peroxisome proliferator-activated receptor γ (PPARγ) activation, but unfortunately it causes bone loss in animals and humans. A previous study showed that prolyl hydroxylase domain protein (PHD) plays a role in rosiglitazone-induced adipocyte differentiation. Based on the inverse relationship between adipocyte and osteoblast differentiation, we investigated whether PHD is involved in the effects of rosiglitazone on osteoblast differentiation. Rosiglitazone inhibited osteoblast differentiation in a concentration-dependent manner, and in parallel induced three PHD isoforms (PHD1, 2, and 3). PHD inhibitors and knockdown of each isoform prevented the inhibitory effects of rosiglitazone on osteoblast differentiation and increased the expression of Runx2, a transcription factor essential for osteoblastogenesis. MG-132, a proteasomal inhibitor also prevented the rosiglitazone-induced degradation of Runx2. Furthermore, both increased PHD isoform expressions and reduced osteoblast differentiation by rosiglitazone were prevented by PPARγ antagonists, indicating these effects were mediated via PPARγ activation. In vivo oral administration of rosiglitazone to female ICR mice for 8 weeks reduced bone mineral densities and plasma alkaline phosphatase (ALP) activity, and increased PHD expression in femoral primary bone marrow cells and the ubiquitination of Runx2. Together, this suggests that the rosiglitazone-induced suppression of osteoblast differentiation is at least partly induced via PPARγ-mediated PHD induction and subsequent promotion of the ubiquitination and degradation of Runx2.

COX2 is involved in hypoxia-induced TNF-α expression in osteoblast

Bone regeneration involves a series of events in a coordinated manner, including recruitment of mesenchymal stem cells, induction of immune response, inflammatory activity and vascular ingrowth. The microenvironment of bone regeneration is hypoxic. Low oxygen tension (hypoxia) promotes the upregulation of several signaling molecules. The primary mediating factor is the hypoxia-inducible factor-1 (HIF-1). Hypoxia stimulates the expression of a variety of cytokines from inflammatory cells, fibroblasts, endothelial cells, and osteoblasts. TNF-α is a key proinflammatory cytokine. The molecular events involved in osteoblast dysfunction under hypoxia are not fully understood. This study determined the effects of hypoxia on TNF-α in osteoblasts, and molecular mechanisms were explored. We observed that hypoxia induced TNF-α expression in a time-dependent manner in osteoblasts. Experiments using a potent HIF-1α activator DFO demonstrated that hypoxia-induced TNF-α was mediated by HIF-1-α. In addition, this study showed that hypoxia activated cyclooxygenase-2 (COX2) expression along with TNF-α. Inhibition experiments using COX2 inhibitor N398 indicated that COX2 was involved in hypoxia-mediated TNF-α expression, and this observation was further confirmed by Small interfering RNA against COX2. On the other hand, TNF-α didn’t lead to the activation of COX2 expression. We conclude that COX2 is involved in hypoxia-induced TNF-α expression in osteoblast.

In vivo and in vitro characteristic of HIF-1α and relative genes in ischemic femoral head necrosis

BACKGROUND

Legg-Calvé-Perthes Disease (Perthes' disease) is a childhood hip disorder initiated by ischemic necrosis of the growing femoral head. So far, the etiology and pathogenesis of Perthes' disease is poorly understood.

MATERIALS AND METHODS

Avascular osteonecrosis rat model was established to mimic the pathophysiological changes of femoral head necrosis. The chondrocytes of newborn Sprague-Dawley rats were isolated and cultured in hypoxic and normoxic condition. The expression characteristic of the hypoxia-inducible factor-1 alpha (HIF-1α) was evaluated both in vivo and in vitro models. Vascular endothelial growth factor (VEGF) and apoptotic genes in chondrocytes treated with normoxia and hypoxia were also studied.

RESULTS

HIF-1α expression increased greatly after ischemic operation and kept at relative high level in the arthromeningitis stage and declined in the stages of osteonecrosis and reconstruction. The HIF-1α mRNA levels of chondrocytes incubated at hypoxia were significantly higher than the cells treated with normoxia at 24 and 72 hours. Hypoxia inhibited VEGF expression; chondrocytes could oppose this inhibition manifested by the increasing of VEGF mRNA level after 72 hours hypoxia. The expression of apoptotic genes, Casp3, Casp8 and Casp9, elevated in chondrocytes after hypoxia with time differences.

CONCLUSION

Hypoxia might be an etiological factor for femoral head necrosis, HIF-1α, VEGF as well as apoptotic genes participated the pathophysiological process of ischemic osteonecrosis.

Hypoxia signalling manipulation for bone regeneration

Hypoxia-inducible factor (HIF) signalling is intricately involved in coupling angiogenesis and osteogenesis during bone development and repair. Activation of HIFs in response to a hypoxic bone micro-environment stimulates the transcription of multiple genes with effects on angiogenesis, precursor cell recruitment and differentiation. Substantial progress has been made in our understanding of the molecular mechanisms by which oxygen content regulates the levels and activity of HIFs. In particular, the discovery of the role of oxygen-dependent hydroxylase enzymes in modulating the activity of HIF-1α has sparked interest in potentially promising therapeutic strategies in multiple clinical fields and most recently bone healing. Several small molecules, termed hypoxia mimics, have been identified as activators of the HIF pathway and have demonstrated augmentation of both bone vascularity and bone regeneration in vivo. In this review we discuss key elements of the hypoxic signalling pathway and its role in bone regeneration. Current strategies for the manipulation of this pathway for enhancing bone repair are presented with an emphasis on recent pre-clinical in vivo investigations. These findings suggest promising approaches for the development of therapies to improve bone repair and tissue engineering strategies.

HIF-1α disturbs osteoblasts and osteoclasts coupling in bone remodeling by up-regulating OPG expression

Hypoxia-inducible factor 1α (HIF-1α) is one of the master regulators of hypoxia reactions, playing an important role in bone modeling, remodeling, and homeostasis. And overexpression of HIF-1α in mature osteoblasts through conditional deletion of the von Hippel-Lindau (VHL) gene profoundly increases angiogenesis and osteogenesis. Studies showed that mice with osteoblasts lacking Vhl had a high level of Hif-1α and increased bone mass and density. On the contrary, Hif-1α conditional knockout mice had decreased bone mass and density. Our in vitro study showed that osteoprotegerin (OPG), an essential regulator of osteoclastic activity, can be upregulated by HIF-1α and in turn downregulate the resorption activity of osteoclasts. We showed that HIF-1α may directly bind to the upstream site of OPG and enhance its expression. Our study suggested that a novel mechanism, which works via OPG signaling, may mediate the function of HIF-1α in bone remodeling.

HIF-1α regulates bone formation after osteogenic mechanical loading

HIF-1 is a transcription factor typically associated with angiogenic gene transcription under hypoxic conditions. In this study, mice with HIF-1α deleted in the osteoblast lineage (ΔHIF-1α) were subjected to damaging or non-damaging mechanical loading known to produce woven or lamellar bone, respectively, at the ulnar diaphysis. By microCT, ΔHIF-1α mice produced significantly less woven bone than wild type (WT) mice 7days after damaging loading. This decrease in woven bone volume and extent was accompanied by a significant decrease in vascularity measured by immunohistochemistry against vWF. Additionally, osteocytes, rather than osteoblasts, appear to be the main bone cell expressing HIF-1α following damaging loading. In contrast, 10days after non-damaging mechanical loading, dynamic histomorphometry measurements demonstrated no impairment in loading-induced lamellar bone formation in ΔHIF-1α mice. In fact, both non-loaded and loaded ulnae from ΔHIF-1α mice had increased bone formation compared with WT ulnae. When comparing the relative increase in periosteal bone formation in loaded vs. non-loaded ulnae, it was not different between ΔHIF-1α mice and controls. There were no significant differences observed between WT and ΔHIF-1α mice in endosteal bone formation parameters. The increases in periosteal lamellar bone formation in ΔHIF-1α mice are attributed to non-angiogenic effects of the knockout. In conclusion, these results demonstrate that HIF-1α is a pro-osteogenic factor for woven bone formation after damaging loading, but an anti-osteogenic factor for lamellar bone formation under basal conditions and after non-damaging loading.

Differential expression of vascular endothelial growth factor in human fetal skeletal site-specific tissues: Mandible versus femur

Vascular endothelial growth factor (VEGF) is a well-known mediator that signals through pathways in angiogenesis and osteogenesis. Angiogenesis and bone formation are coupled during either skeletal development or bone remodeling and repair occurring in postnatal life. In this study, we examined for the first time the expression of VEGF in human fetal mandibular and femoral bone in comparison with the respective adult tissues. Similarly to other craniofacial bones, but at variance with the axial and appendicular skeleton, during development mandible does not arise from mesoderm but neural crest cells of the neuroectoderm germ layer, and undergoes intramembranous instead of endochondral ossification. By quantitative real-time PCR technique, we could show that VEGF gene expression levels were significantly higher in fetal than in adult samples, especially in femoral tissue. Western blotting analysis confirmed higher protein expression of VEGF in the fetal femur respect to the mandible. Moreover, immunohistochemistry revealed that in both fetal tissues VEGF expression was mainly localized in pre- and osteoblasts. Differential expression of VEGF in femoral and mandibular bone tissues could be related to their different structure, function and development during organogenesis.

Diallyl trisulfide inhibits migration, invasion and angiogenesis of human colon cancer HT-29 cells and umbilical vein endothelial cells, and suppresses murine xenograft tumour growth

Angiogenesis inhibitors are beneficial for the prevention and treatment of angiogenesis-dependent diseases including cancer. We examined the cytotoxic, anti-metastatic, anti-cancer and anti-angiogenic effects of diallyl trisulfide (DATS). In HT29 cells, DATS inhibited migration and invasion through the inhibition of focal adhesion kinase (FAK), extracellular signal-regulated kinase, c-Jun N-terminal kinase and p38 which was associated with inhibition of matrix metalloproteinases-2, -7 and -9 and VEGF. In human umbilical vein endothelial cells (HUVEC), DATS inhibited the migration and angiogenesis through FAK, Src and Ras. DATS also inhibited the secretion of VEGF. The capillary-like tube structure formation and migration by HUVEC was inhibited by DATS. The chicken egg chorioallantoic membrane (CAM) assay indicated that DATS treatment inhibited ex-vivo angiogenesis. We investigated the anti-tumour effects of DATS against human colon cancer xenografts in BALB/c^nu/nu mice and its anti-angiogenic activity in vivo. In this in-vivo study, DATS also inhibited the tumour growth, tumour weight and angiogenesis (decreased the levels of haemoglobin) in HT29 cells. In conclusion, the present results suggest that the inhibition of angiogenesis may be an important mechanism in colon cancer chemotherapy by DATS.

Endogenous bone regeneration is dependent upon a dynamic oxygen event

Amputation of the digit tip within the terminal phalangeal bone of rodents, monkeys, and humans results in near-perfect regeneration of bone and surrounding tissues; however, amputations at a more proximal level fail to produce the same regenerative result. Digit regeneration is a coordinated, multifaceted process that incorporates signaling from bioactive growth factors both in the tissue matrix and from several different cell populations. To elucidate the mechanisms involved in bone regeneration we developed a novel multi-tissue slice-culture model that regenerates bone ex vivo via direct ossification. Our study provides an integrated multi-tissue system for bone and digit regeneration and allows us to circumvent experimental limitations that exist in vivo. We used this slice-culture model to evaluate the influence of oxygen on regenerating bone. Micro-computed tomography (µCT) and histological analysis revealed that the regenerative response of the digit is facilitated in part by a dynamic oxygen event, in which mutually exclusive high and low oxygen microenvironments exist and vacillate in a coordinated fashion during regeneration. Areas of increased oxygen are initially seen in the marrow and then surrounding areas of vasculature in the regenerating digit. Major hypoxic events are seen at 7 days postamputation (DPA 7) in the marrow and again at DPA 12 in the blastema, and manipulation of oxygen tensions during these hypoxic phases can shift the dynamics of digit regeneration. Oxygen increased to 21% oxygen tension can either accelerate or attenuate bone mineralization in a stage-specific manner in the regenerative timeline. These studies not only reveal a circumscribed frame of oxygen influence during bone regeneration, but also suggest that oxygen may be one of the primary signaling influences during regeneration.

Conditional disruption of the prolyl hydroxylase domain-containing protein 2 (Phd2) gene defines its key role in skeletal development

We have previously shown that the increase in osterix (Osx) expression during osteoblast maturation is dependent on the activity of the prolyl hydroxylase domain-containing protein 2 (Phd2), a key regulator of protein levels of the hypoxia-inducible factor family proteins in many tissues. In this study, we generated conditional Phd2 knockout mice (cKO) in osteoblast lineage cells by crossing floxed Phd2 mice with a Col1α2-iCre line to investigate the function of Phd2 in vivo. The cKO mice developed short stature and premature death at 12 to 14 weeks of age. Bone mineral content, bone area, and bone mineral density were decreased in femurs and tibias, but not vertebrae of the cKO mice compared to WT mice. The total volume (TV), bone volume (BV), and bone volume fraction (BV/TV) in the femoral trabecular bones of cKO mice were significantly decreased. Cross-sectional area of the femoral mid-diaphysis was also reduced in the cKO mice. The reduced bone size and trabecular bone volume in the cKO mice were a result of impaired bone formation but not bone resorption as revealed by dynamic histomorphometric analyses. Bone marrow stromal cells derived from cKO mice formed fewer and smaller nodules when cultured with mineralization medium. Quantitative RT-PCR and immunohistochemistry detected reduced expression of Osx, osteocalcin, and bone sialoprotein in cKO bone cells. These data indicate that Phd2 plays an important role in regulating bone formation in part by modulating expression of Osx and bone formation marker genes.

EPO promotes bone repair through enhanced cartilaginous callus formation and angiogenesis

Erythropoietin (EPO)/erythropoietin receptor (EPOR) signaling is involved in the development and regeneration of several non-hematopoietic tissues including the skeleton. EPO is identified as a downstream target of the hypoxia inducible factor-α (HIF-α) pathway. It is shown that EPO exerts a positive role in bone repair, however, the underlying cellular and molecular mechanisms remain unclear. In the present study we show that EPO and EPOR are expressed in the proliferating, pre-hypertrophic and hypertrophic zone of the developing mouse growth plates as well as in the cartilaginous callus of the healing bone. The proliferation rate of chondrocytes is increased under EPO treatment, while this effect is decreased following siRNA mediated knockdown of EPOR in chondrocytes. EPO treatment increases biosynthesis of proteoglycan, accompanied by up-regulation of chondrogenic marker genes including SOX9, SOX5, SOX6, collagen type 2, and aggrecan. The effects are inhibited by knockdown of EPOR. Blockage of the endogenous EPO in chondrocytes also impaired the chondrogenic differentiation. In addition, EPO promotes metatarsal endothelial sprouting in vitro. This coincides with the in vivo data that local delivery of EPO increases vascularity at the mid-stage of bone healing (day 14). In a mouse femoral fracture model, EPO promotes cartilaginous callus formation at days 7 and 14, and enhances bone healing at day 28 indexed by improved X-ray score and micro-CT analysis of microstructure of new bone regenerates, which results in improved biomechanical properties. Our results indicate that EPO enhances chondrogenic and angiogenic responses during bone repair. EPO's function on chondrocyte proliferation and differentiation is at least partially mediated by its receptor EPOR. EPO may serve as a therapeutic agent to facilitate skeletal regeneration.

HIF-1 α had pivotal effects on downregulation of miR-210 decreasing viability and inducing apoptosis in hypoxic chondrocytes

Hypoxia-inducible factor 1-alpha (HIF-1 α ) and some microRNA (miRNAs) play pivotal roles in response to hypoxia-related physiologic and pathophysiologic responses. Up to date, the regulatory mechanisms of these molecules were largely unknown in chondrocytes. In this study, to study the mechanisms of degradation and homeostasis of chondrocytes, the effects of miRNAs and HIF-1 α on chondrocytes in physiologic environment were investigated. We found that the overexpression of miR-210 and HIF-1 α was present on hypoxia in C28/I2 human chondrocytes significantly by qRT-PCR and western plot. Further study displayed that miR-210 played positive role as a promoter in regulation and its regulated molecules (bcl-xl and PHD-2) in C28/I2 cells on hypoxia by silenced miR-210, silenced HIF-1 α , and adding miR-210. Moreover, downregulated miR-210 could significantly repress the viability and increase the apoptosis in C28/I2 cells on hypoxia, compared to those on normoxia. Furthermore, miR-210 could not modulate viability and apoptosis in C28/I2 cells with the HIF-1 α knockdown on hypoxia and normoxia. Taken together, this study demonstrated that the MiR-210 was involved in an HIF-1 α -dependent way in C28/I2 human chondrocytes for the first time. It also suggested that miR-210 downregulation decreased viability and induced apoptosis in hypoxic chondrocytes depending on HIF-1 α .

Dimethyloxaloylglycine increases the bone healing capacity of adipose-derived stem cells by promoting osteogenic differentiation and angiogenic potential

Hypoxia inducible factor-1α (HIF-1α) plays an important role in angiogenesis-osteogenesis coupling during bone regeneration, which can enhance the bone healing capacity of mesenchymal stem cells (MSCs) by improving their osteogenic and angiogenic activities. Previous studies transduced the HIF-1α gene into MSCs with lentivirus vectors to improve their bone healing capacity. However, the risks due to lentivirus vectors, such as tumorigenesis, should be considered before clinical application. Dimethyloxaloylglycine (DMOG) is a cell-permeable prolyl-4-hydroxylase inhibitor, which can activate the expression of HIF-1α in cells at normal oxygen tension. Therefore, DMOG is expected to be an alternative strategy for enhancing HIF-1α expression in cells. In this study, we explored the osteogenic and angiogenic activities of adipose-derived stem cells (ASCs) treated with different concentrations of DMOG in vitro, and the bone healing capacity of DMOG-treated ASCs combined with hydrogels for treating critical-sized calvarial defects in rats. The results showed that DMOG had no obvious cytotoxic effects on ASCs and could inhibit the death of ASCs induced by serum deprivation. DMOG markedly increased vascular endothelial growth factor production in ASCs in a dose-dependent manner and improved the osteogenic differentiation potential of ASCs by activating the expression of HIF-1α. Rats with critical-sized calvarial defects treated with hydrogels containing DMOG-treated ASCs had more bone regeneration and new vessel formation than the other groups. Therefore, we believe that DMOG enhanced the angiogenic and osteogenic activity of ASCs by activating the expression of HIF-1α, thereby improving the bone healing capacity of ASCs in rat critical-sized calvarial defects.

BMP signaling in mesenchymal stem cell differentiation and bone formation

Bone morphogenetic proteins (BMPs) are members of the TGF-β superfamily and have diverse functions during development and organogenesis. BMPs play a major role in skeletal development and bone formation, and disruptions in BMP signaling cause a variety of skeletal and extraskeletal anomalies. Several knockout models have provided insight into the mechanisms responsible for these phenotypes. Proper bone formation requires the differentiation of osteoblasts from mesenchymal stem cell (MSC) precursors, a process mediated in part by BMP signaling. Multiple BMPs, including BMP2, BMP6, BMP7 and BMP9, promote osteoblastic differentiation of MSCs both in vitro and in vivo. BMP9 is one of the most osteogenic BMPs yet is a poorly characterized member of the BMP family. Several studies demonstrate that the mechanisms controlling BMP9-mediated osteogenesis differ from other osteogenic BMPs, but little is known about these specific mechanisms. Several pathways critical to BMP9-mediated osteogenesis are also important in the differentiation of other cell lineages, including adipocytes and chondrocytes. BMP9 has also demonstrated translational promise in spinal fusion and bone fracture repair. This review will summarize our current knowledge of BMP-mediated osteogenesis, with a focus on BMP9, by presenting recently completed work which may help us to further elucidate these pathways.

HIF-1α inhibits Wnt signaling pathway by activating Sost expression in osteoblasts

The nature of the cellular and molecular mechanisms for the transition of avascular cartilage replacement with bone during endochondral ossification remains poorly understood. One of the driving forces is hypoxia. As a master regulator of hypoxia, hypoxia-inducible factor-1α (HIF-1α) has been reported to couple angiogenesis to osteogenesis. Our recent study has demonstrated that osteoblast growth is inhibited under hypoxia and that HIF-1α cooperates with Osterix (Osx) to inhibit Wnt pathway. However, molecular mechanisms for inhibitory effects of HIF-1α on Wnt pathway are not well understood. In this study, our quantitative RT-PCR results revealed that the expression of a Wnt antagonist Sclerostin (Sost) was upregulated in osteoblasts during hypoxia while HIF-1α was upregulated. Treatment of desferrioxamine (DFO), a HIF-1α activator, led to further increase of Sost expression, suggesting that HIF-1α may activate Sost expression. The regulation of Sost gene expression by HIF-1α was then investigated. We performed loss-of-function experiments to examine Sost expression by using siRNA approach against HIF-1α, and found that the inhibition of HIF-1α by siRNA in osteoblasts led to the decrease of Sost expression. To address transcriptional regulation of Sost gene by HIF-1α, transient transfection assay was performed and showed that HIF-1α activated Sost-1 kb promoter reporter activity in a dose-dependent manner. To narrow down the minimal region of Sost promoter activated by HIF-1α, we generated a series of deletion mutants of Sost constructs. It was demonstrated that Sost-260 was the minimal region of Sost promoter for HIF-1α activation and that Sost-106 construct, which lack hypoxia response element, abolished HIF-1α-mediated Sost reporter activation. Gel shift assay showed that HIF-1 bound to the promoter sequence of Sost directly. These findings support our hypothesis that HIF-1α activates Sost expression. This study provides a novel molecular mechanism through which HIF-1α inhibits Wnt signaling in osteoblasts.

HIF-1α transgenic bone marrow cells can promote tissue repair in cases of corticosteroid-induced osteonecrosis of the femoral head in rabbits

Although corticosteroid-induced osteonecrosis of the femoral head (ONFH) is common, the treatment for it remains limited and largely ineffective. We examined whether implantation of hypoxia inducible factor-1α (HIF-1α) transgenic bone marrow cells (BMCs) can promote the repair of the necrotic area of corticosteroid-induced ONFH. In this study, we confirmed that HIF-1α gene transfection could enhance mRNA expression of osteogenic genes in BMCs in vitro. Alkaline phosphatase activity assay and alizarin red-S staining indicated HIF-1α transgenic BMCs had enhanced osteogenic differentiation capacity in vitro. Furthermore, enzyme linked immunosorbent assay (ELISA) for VEGF revealed HIF-1α transgenic BMCs secreted more VEGF as compared to normal BMCs. An experimental rabbit model of early-stage corticosteroid-induced ONFH was established and used for an evaluation of cytotherapy. Transplantation of HIF-1α transgenic BMCs dramatically improved the bone regeneration of the necrotic area of the femoral head. The number and volume of blood vessel were significantly increased in the necrotic area of the femoral head compared to the control groups. These results support HIF-1α transgenic BMCs have enhanced osteogenic and angiogenic activity in vitro and in vivo. Transplantation of HIF-1α transgenic BMCs can potentially promote the repair of the necrotic area of corticosteroid-induced ONFH.